Both of a hub axle and an outer race of the wheel support bearing assembly of a hub unit type are manufactured through a machining process that is effected subsequent to a forging process. The forging process is generally carried out by heating to about 1,100° C., a carbon steel bar containing, for example, 0.4 to 0.8% of carbon after such carbon steel bar has been cut in a sectional direction, followed by upsetting, preforming, finish-forging and boring.
Since the hub axle and the outer race are each forged in the manner described above, the fiber flow exhibited by the carbon steel bar for the hub axle and that for the outer race after the forging process represent such as shown in FIG. 11 and FIG. 13, respectively. In those figures, the finished contour of the hub axle 81 and that of the outer race 84, both after the machining, are shown by broken lines, respectively.
FIG. 12 illustrates an enlarged sectional view of a portion A of FIG. 11, showing a raceway surface 90 of the hub axle 81, which has been formed by machining. FIGS. 14A and 14B illustrate enlarged sectional views of portions A and B of FIG. 13, showing raceway surfaces 92 and 93 of the outer race 84, which have been formed by machining, respectively. In FIGS. 12 and 14, each of the raceway surfaces 90, 92 and 93 has the curvature with the center O. In the range of the curvature from an edge portion X in the bottom portion of each of the groove-shaped raceway surfaces 90, 92 and 93 where the curvature starts to another edge portion Y in the shoulder portion of each surface, the straight line L is drawn so as to connect between the center of curvature O and the point P at which the section of the fiber flow F precipitates and, on the other hand, the tangential line T is drawn to pass through the point P of intersection between the straight line L and each of the raceway surfaces. The angle α between the tangential line T1 of the fiber flow F and the tangential line T is obtained. This angle is defined as the angle α of the fiber flow F relative to the respective raceway surface.
This angle α of the fiber flow is related to the magnitude of the machining allowance (the difference between the shape of the forged raceway surface and the shape of the machined raceway surface for finish. There is a tendency that increase of the machining allowance is accompanied by increase of the angle α of the fiber flow. The angle α of the fiber flow varies between 15°<α<20° in the case of the hub axle 81 and between 15°<α<80° in the case of the outer race 84 which shows a large machining allowance.
The fiber flow F is a flow of material which is induced during formation of the bar, and impurities left unremoved during the steel manufacture somewhat exist in the bar. Those impurities exist along the fiber flow F. In general, under the normal lubricating condition, the fatigue life of the rolling bearing assembly is affected largely by impurities, particularly those of an oxidizing system, contained in the material. It is generally said that the duration of life decreases if the impurities are large and long and/or many in number.
While based on the result of experiments conducted using test pieces, there is a correlation between the angle of the fiber flow relative to the raceway surface and the life of the rolling bearing assembly, and it is known that with increase of the angle, the duration of life of the rolling bearing assembly decreases. Also, even in the wheel support bearing assembly, as compared with the manufacture thereof starting from the use of a pipe material, the manufacture thereof starting from the use of a rod material is said to result in lessening the possibility of the fiber flow being isolated or cut (See the Japanese Patent Publication No. 5-66215).
However, since the standard wheel support bearing assembly satisfies the rolling fatigue life that is currently required, no attention is basically paid to the fiber flow and the shape of the material before the machining is determined only by the reason of easiness to forge. However, the shape of the material before the machining, which is easy to forge, involves a large amount of the machining allowance and requires an increased number of machining steps. As a result thereof, the cost of manufacturing is increased and it does not lead to reduction of the cost of the product. On the other hand, even though the rolling life of the standard wheel support bearing assembly is currently satisfactory, it may be expected in the future that demands may increase to render the wheel support bearing assembly, that is, an automobile component part that is used under severe conditions, to have a longer rolling lifetime. Although the Japanese Patent Publication No. 5-66215 referred to above describes that as compared with the manufacture of the wheel support bearing assembly starting from the use of a pipe material, the manufacture thereof starting from the use of a round rod material can result in lessening the possibility of the fiber flow being isolated or cut, neither the ingenuity for minimization of the isolation or cut of the fiber flow nor any consideration on the angle of the fiber flow has been made. Also, the manufacturing method disclosed in the above mentioned patent publication is directed to a method of manufacturing a collarless outer race, and it is not clear as to whether the method disclosed can be extended or applied to the hub axle and the collared outer race.